Thousands of supporters of Shi'ite cleric Moqtada al-Sadr marched through the holy Iraqi city of Najaf in an angry funeral procession after a senior Sadr aide was killed by a US soldier today.
Chanting "No to America" and carrying placards decrying US occupation, mourners, including black-robed clerics, carried the coffin of Saheb al-Amiri through the streets.
Members of Sadr's group accused the Americans of provoking a confrontation and demanded a government inquiry.
Najaf, home to Iraq's top Shi'ite clerics, was the site of a 2004 rebellion against US forces by militias loyal to Sadr, who also has power bases in Baghdad.
Another uprising by Sadr Mehdi Army militias would be a major headache for the US military, which has 135,000 troops in a country gripped by Shi'ite-Sunni strife.
Sadr officials and US military commanders gave conflicting versions of the activities of Amiri.
Sadr officials said US forces stormed his home at dawn and killed him in front of his wife and children. They said Amiri was a lawyer who headed a charity for orphans and the poor and was not part of the Mehdi Army.
"What happened was a crime. It comes on top of other crimes committed by occupation forces in Iraq," Nassar al-Rubaei, head of the Sadr bloc in parliament, told reporters in Baghdad.
US military spokesman Major General William Caldwell said Amiri was an "improvised explosive device facilitator" and that he was implicated in a bomb attack on a police chief in October.
He said he was shot dead in his home near Najaf, during an Iraqi-led raid, by a US military adviser who saw Amiri point an assault rifle at an Iraqi soldier.
He said US forces had "tremendous amount of information" on Amiri for some time.
"The purpose of going after him is because of the illegal activities that he was conducting, not because he was associated to any particular organisation," Maj Gen Caldwell told reporters.
US commanders have put pressure on Shi'ite Prime Minister Nuri al-Maliki to crack down on Shi'ite militias, which have ties to his coalition.